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</head>
<body class="article toc2 toc-left">
<div id="header">
<h1>Commands Example</h1>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_before_we_start">Before we start</a>
<ul class="sectlevel2">
<li><a href="#_icommand">ICommand</a></li>
</ul>
</li>
<li><a href="#_solution_1_use_reactiveui">Solution 1: Use ReactiveUI</a>
<ul class="sectlevel2">
<li><a href="#_step_1_create_a_new_avalonia_mvvm_application">Step 1: Create a new Avalonia MVVM Application</a></li>
<li><a href="#_step_2_add_a_viewmodel">Step 2: Add a ViewModel</a></li>
<li><a href="#_step_3_create_the_open_the_pod_bay_doors_command">Step 3: Create the "Open the POD bay doors" - Command</a></li>
<li><a href="#_step_4_create_the_fellow_robot_command">Step 4: Create the "Fellow Robot" - Command</a></li>
<li><a href="#_step_5_create_open_the_pod_bay_sequence_async_command">Step 5: Create "Open the Pod Bay sequence" - Async Command</a></li>
<li><a href="#Solution1-Step6">Step 6: Setting up the User Interface</a></li>
<li><a href="#_step_7_see_it_in_action">Step 7: See it in action</a></li>
</ul>
</li>
<li><a href="#_solution_2_use_communitytookit_mvvm">Solution 2: Use CommunityTookit-MVVM</a>
<ul class="sectlevel2">
<li><a href="#_step_1_add_a_viewmodel">Step 1: Add a ViewModel</a></li>
<li><a href="#_step_2_create_the_open_the_pod_bay_doors_command">Step 2: Create the "Open the POD bay doors" - Command</a></li>
<li><a href="#_step_3_create_the_fellow_robot_command">Step 3: Create the "Fellow Robot" - Command</a></li>
<li><a href="#_step_4_create_open_the_pod_bay_sequence_async_command">Step 4: Create "Open the Pod Bay sequence" - Async Command</a></li>
<li><a href="#_step_5_setting_up_the_user_interface">Step 5: Setting up the User Interface</a></li>
<li><a href="#_step_6_see_it_in_action">Step 6: See it in action</a></li>
</ul>
</li>
<li><a href="#_related">Related</a></li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>This example will show you how you can use <code>Commands</code> to invoke methods in your <code>ViewModel</code> from your user interface.</p>
</div>
<!-- toc disabled -->
<h3 id="_difficulty" class="discrete">Difficulty</h3>
<div class="paragraph">
<p>🐣 Beginner 🐣</p>
</div>
<h3 id="_buzz_words" class="discrete">Buzz-Words</h3>
<div class="paragraph">
<p>Command, ICommand, CommandParameter, MVVM, ReactiveCommand, RelayCommand, async-await</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_before_we_start">Before we start</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In this sample we assume that you know what a <code>Binding</code> is and that you have a basic knowledge of the <code>MVVM</code> concepts. Moreover you should already know how to create a new Avalonia project.</p>
</div>
<div class="paragraph">
<p>In case you want to refresh your knowledge, visit the <a href="../BasicMvvmSample">[Basic MVVM Sample]</a>.</p>
</div>
<div class="sect2">
<h3 id="_icommand">ICommand</h3>
<div class="paragraph">
<p>If a class implements the <a href="https://docs.microsoft.com/en-us/dotnet/api/system.windows.input.icommand?view=net-6.0">[<code>ICommand</code>]</a> interface it can be used to execute a <code>Method</code> in your <code>Model</code> or <code>ViewModel</code>.  Moreover, the interface allows you to easily enable or disable the <code>Control</code> (most often a <code>Button</code>), that your <code>Command</code> is bound to.</p>
</div>
<div class="paragraph">
<p>The interface has three members:</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1">CanExecute(Object)</dt>
<dd>
<p>Defines the method that determines whether the command can execute in its current state.</p>
</dd>
<dt class="hdlist1">Execute(Object)</dt>
<dd>
<p>Defines the method to be called when the command is invoked.</p>
</dd>
<dt class="hdlist1">CanExecuteChanged-Event</dt>
<dd>
<p>Occurs when changes occur that affect whether or not the command should execute.</p>
</dd>
</dl>
</div>
<div class="paragraph">
<p><em>Definitions copied from <a href="https://docs.microsoft.com/en-us/dotnet/api/system.windows.input.icommand?view=net-6.0" class="bare">https://docs.microsoft.com/en-us/dotnet/api/system.windows.input.icommand?view=net-6.0</a></em></p>
</div>
<div class="sect3">
<h4 id="_reactivecommand_and_relaycommand">ReactiveCommand and RelayCommand</h4>
<div class="paragraph">
<p>There are many MVVM-packages available that already have implemented the interface for us. In this sample we will use these two options:</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1">ReactiveUI</dt>
<dd>
<p>This library ships <a href="https://www.reactiveui.net/docs/handbook/commands/">[<code>ReactiveCommand</code>]</a>.</p>
</dd>
<dt class="hdlist1">CommunityToolkit-MVVM</dt>
<dd>
<p>This library ships <a href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/relaycommand">[<code>RelayCommand</code>]</a> and <a href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/asyncrelaycommand">[<code>AsyncRelayCommand</code>]</a>. In addition it provides a <a href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/relaycommand">[<code>source generator</code>]</a> to reduce the boilerplate code needed.</p>
</dd>
</dl>
</div>
</div>
<div class="sect3">
<h4 id="_commands_and_long_running_tasks">Commands and long running Tasks</h4>
<div class="paragraph">
<p>Some <code>MVVM</code>-libs, provide an async implementations of the <code>ICommand</code> interface. That way we can execute long running tasks from our UI without blocking the UI.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_solution_1_use_reactiveui">Solution 1: Use ReactiveUI</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In this sample, we will give commands to HAL-9000, a stubborn computer, in a homage to the famous "Open the POD bay doors" scene from the movie "2001 A Space Odyssey".</p>
</div>
<div class="sect2">
<h3 id="_step_1_create_a_new_avalonia_mvvm_application">Step 1: Create a new Avalonia MVVM Application</h3>
<div class="paragraph">
<p>Create a new Project using the <code>Avalonia MVVM Application</code>-Template</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>In case you don&#8217;t have ReactiveUI installed, add it to your nuget-packages and make sure you have added this line in <code>Program.cs</code>:</p>
</div>
<div class="listingblock">
<div class="title">Program.cs</div>
<div class="content">
<pre class="highlight"><code class="language-diff" data-lang="diff">    public static AppBuilder BuildAvaloniaApp()
        =&gt; AppBuilder.Configure&lt;App&gt;()
            .UsePlatformDetect()
            .LogToTrace()
+           .UseReactiveUI();</code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_step_2_add_a_viewmodel">Step 2: Add a ViewModel</h3>
<div class="paragraph">
<p>Add a new class into the <code>ViewModels</code>-folder called <code>ReactiveUiCommandsViewModel</code>. This class should inherit <code>ViewModelBase</code> or <code>ReactiveObject</code>.
Now open the file <code>ViewModels ► MainWindowViewModel.cs</code> or <code>MainViewModel</code> and add a new instance of the newly created class to it.</p>
</div>
<div class="listingblock">
<div class="title">MainViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public class MainViewModel : ViewModelBase
{
    public ReactiveUiCommandsViewModel ReactiveUiCommandsViewModel { get; } = new ReactiveUiCommandsViewModel();
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>We want to store a record of our conversation with HAL-9000.  We use an <a href="https://docs.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.observablecollection-1?view=net-6.0"><code>[ObservableCollection]</code></a>, because this collection will notify our UI whenever a line was added or removed. In the file <code>ReactiveUiCommandsViewModel</code> add the following lines:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
///  This collection will store what the computer said
/// &lt;/summary&gt;
public ObservableCollection&lt;string&gt; ConversationLog { get; } = new ObservableCollection&lt;string&gt;();

// Just a helper to add content to ConversationLog
private void AddToConvo(string content)
{
    ConversationLog.Add(content);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Since HAL-9000 is not likely to listen to us, we also want to have the suggestion come
from a fellow robot. We want a field to put in the robot&#8217;s name. Let&#8217;s add a property:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">// Backing field for RobotName
private string? _RobotName;

/// &lt;summary&gt;
/// The name of a robot. If the name is null or empty, there is no other robot present.
/// &lt;/summary&gt;
public string? RobotName
{
    get =&gt; _RobotName;
    set =&gt; this.RaiseAndSetIfChanged(ref _RobotName, value);
}</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_3_create_the_open_the_pod_bay_doors_command">Step 3: Create the "Open the POD bay doors" - Command</h3>
<div class="paragraph">
<p>In the <code>ViewModel`</code> add a new read-only property of type <code>ICommand</code>. We use the interface instead of the specialized class, because it is more flexible. For example You can easily replace it with an async implementation if you need to.</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
/// This command will ask HAL-9000 to open the pod bay doors
/// &lt;/summary&gt;
public ICommand OpenThePodBayDoorsDirectCommand { get; }</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now we will create a method which will be executed when the <code>Command</code> is invoked:</p>
</div>
<div class="paragraph">
<p>HAL responds to the human named "Dave" that he can&#8217;t do that.
This is a famous line from the movie.</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">// The method that will be executed when the command is invoked
private void OpenThePodBayDoors()
{
    ConversationLog.Clear();
    AddToConvo( "I'm sorry, Dave, I'm afraid I can't do that.");
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Last but not least we need to initialize our <code>Command</code> before we can use it. We will do this in our <a href="https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/constructors"><code>[Constructor]</code></a> as shown below. We create our Commands using <a href="https://www.reactiveui.net/docs/handbook/commands/"><code>[ReactiveCommand.Create())]</code></a> method which will take a method or a lambda as parameter.</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public ReactiveUiCommandsViewModel()
{
    // Init OpenThePodBayDoorsDirectCommand
    OpenThePodBayDoorsDirectCommand = ReactiveCommand.Create(OpenThePodBayDoors);
}</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_4_create_the_fellow_robot_command">Step 4: Create the "Fellow Robot" - Command</h3>
<div class="paragraph">
<p>Perhaps we will have more luck if the command is given by a fellow robot.</p>
</div>
<div class="paragraph">
<p>This <code>Command</code> is very similar to the one above. This time we will use a <code>CommandParameter</code> which is the name of another robot, but in fact it can be any <code>object</code> depending on your needs. We will also use the <code>CommandParameter</code> to enable or disable the <code>Command</code>, depending on the state of the parameter.</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
/// This command will ask HAL to open the pod bay doors, but this time we
/// check that the command is issued by a fellow robot (really any non-null name)
/// &lt;/summary&gt;
public ICommand OpenThePodBayDoorsFellowRobotCommand { get; }</code></pre>
</div>
</div>
<div class="paragraph">
<p>Our method will look like this:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">private void OpenThePodBayDoorsFellowRobot(string? robotName)
{
    ConversationLog.Clear();
    AddToConvo( $"Hello {robotName}, the Pod Bay is open :-)");
}</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
We use string interpolation in the above method. If you want to learn more about it visit <a href="https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated">[Microsoft Docs]</a>.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Again we have to initialize our command in the constructor. We will first create an <a href="https://docs.microsoft.com/en-us/dotnet/api/system.iobservable-1?view=net-6.0">[<code>Observable</code>]</a> using <code>ReactiveUI&#8217;s</code> <a href="https://www.reactiveui.net/docs/handbook/when-any/#watching-single-property">[<code>WhenAnyValue</code>]</a> which will listen to <code>RobotName</code>. The <code>Observable</code> will automatically enable our <code>Command</code> whenever <code>RobotName</code> is not null or empty and disable the <code>Command</code> whenever <code>RobotName</code> is <code>null</code> or empty. Add the following lines to our constructor:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public ReactiveUiCommandsViewModel()
{
    // The IObservable&lt;bool&gt; is needed to enable or disable the command depending on valid parameters
    // The Observable listens to RobotName and will enable the Command if the name is not empty.
    IObservable&lt;bool&gt; canExecuteFellowRobotCommand =
        this.WhenAnyValue(vm =&gt; vm.RobotName, (name) =&gt; !string.IsNullOrEmpty(name));

    OpenThePodBayDoorsFellowRobotCommand =
        ReactiveCommand.Create&lt;string?&gt;(name =&gt; OpenThePodBayDoorsFellowRobot(name), canExecuteFellowRobotCommand);
}</code></pre>
</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
This time we used <code>ReactiveCommand.Create&lt;string?&gt;</code>. the part between <code>&lt;&gt;</code> defines the expected type of our <code>CommandParameter</code> and can be any type you like.
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_step_5_create_open_the_pod_bay_sequence_async_command">Step 5: Create "Open the Pod Bay sequence" - Async Command</h3>
<div class="paragraph">
<p>Many operations have multiple steps and we want to keep the user interface responsive while
we wait for sequence of operations to complete. One way to do this in <code>C#</code> is to use <a href="https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/">[<code>async / await</code>]</a> in combination with <a href="https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task?view=net-6.0">[<code>Tasks</code>]</a> to run things in the background while not blocking the UI.</p>
</div>
<div class="paragraph">
<p>Let&#8217;s add the <code>Command</code> and the <code>Task</code> to our <code>ViewModel</code>:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">// This method is an async Task because opening the pod bay doors can take long time.
// We don't want our UI to become unresponsive.
private async Task OpenThePodBayDoorsAsync()
{
    ConversationLog.Clear();
    AddToConvo( "Preparing to open the Pod Bay...");
    // wait a second
    await Task.Delay(1000);

    AddToConvo( "Depressurizing Airlock...");
    // wait 2 seconds
    await Task.Delay(2000);

    AddToConvo( "Retracting blast doors...");
    // wait 2 more seconds
    await Task.Delay(2000);

    AddToConvo("Pod Bay is open to space!");
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Initiating the async <code>Command</code> in the constructor is very similar to how you init a sync <code>Command</code>, except we use <code>CreateFromTask</code> instead of <code>Create</code>:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public ReactiveUiCommandsViewModel()
{
    // Init OpenThePodBayDoorsAsyncCommand
    OpenThePodBayDoorsAsyncCommand = ReactiveCommand.CreateFromTask(OpenThePodBayDoorsAsync);
}</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
When you execute this command, notice that the <code>Button</code> is disabled until the <code>Task</code> is finished, so you can&#8217;t ask for the pod bay doors to open again while the opening sequence is running. A more complete program could keep track of the state of the Pod Bay and change the button to "close".
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="Solution1-Step6">Step 6: Setting up the User Interface</h3>
<div class="paragraph">
<p>Add a new <code>UserControl</code> into the <code>Views</code> folder called <code>ReactiveUiCommandsSampleView</code>. We will add three <code>Buttons</code> where we bind <code>Button.Command</code> to the <code>ICommands</code> we prepared in the <code>ViewModel</code>. The second <code>Button</code> will also get a binding to <code>Button.CommandParameter</code>.</p>
</div>
<div class="paragraph">
<p>The first <code>Button</code> looks like this:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsSampleView.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;!-- This button will ask HAL to open the doors --&gt;
&lt;Button Command="{Binding OpenThePodBayDoorsDirectCommand}"
        Content="Open the pod bay doors, HAL." /&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>The second <code>Button</code> will have a <code>CommandParameter</code> defined and more over the <code>Content</code> will change dynamically. The <code>Button</code> is rendered next to a <code>TextBox</code>, where we can enter the name of a <code>Robot</code>:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsSampleView.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;!-- This button will ask HAL to open the doors for a fellow robot --&gt;
&lt;!-- We use CommandParameter to provide the name of the robot --&gt;
&lt;!-- Note that the button is automatically disabled if we don't enter a name--&gt;
&lt;StackPanel Orientation="Horizontal" Spacing="5"&gt;
    &lt;TextBox Text="{Binding RobotName}" Watermark="Robot Name" /&gt;
    &lt;Button Command="{Binding OpenThePodBayDoorsFellowRobotCommand}"
            Content="{Binding RobotName, StringFormat='Open the Pod Bay for {0}'}"
            CommandParameter="{Binding RobotName}" /&gt;
&lt;/StackPanel&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>The third <code>Button</code> will bind to our <code>async Command</code>:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsSampleView.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;!-- This button will ask start a multi-step sequence to open the doors --&gt;
&lt;!-- As this is a long operation, the command is async.--&gt;
&lt;!-- HAL inform us about the current progress of the doors.--&gt;
&lt;Button Command="{Binding OpenThePodBayDoorsAsyncCommand}"
        Content="Start Pod Bay Opening Sequence" /&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>We can also bind <code>Button.Command</code> to any <code>Method</code>. To demonstrate that we added another <code>Button</code> which we can use to clear the recent conversation log.</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsSampleView.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;!-- This button will clear the recent conversation log --&gt;
&lt;Button Grid.Column="1"
        VerticalAlignment="Stretch" VerticalContentAlignment="Center"
        Background="Transparent"
        Command="{Binding ConversationLog.Clear}"
        Content="❌"/&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now let&#8217;s put all together. Our <code>View</code> will finally look like this:</p>
</div>
<div class="listingblock">
<div class="title">ReactiveUiCommandsSampleView.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:viewModels="clr-namespace:CommandSample.ViewModels"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
             x:DataType="viewModels:ReactiveUiCommandsViewModel"
             x:Class="CommandSample.Views.ReactiveUiCommandsSampleView"&gt;
    &lt;Design.DataContext&gt;
        &lt;viewModels:ReactiveUiCommandsViewModel/&gt;
    &lt;/Design.DataContext&gt;

    &lt;Grid RowDefinitions="Auto, Auto, *" ColumnDefinitions="Auto, *"&gt;

        &lt;TextBlock Grid.Column="0" Grid.Row="0"
                   Text="Command:"
                   FontWeight="Bold"
                   VerticalAlignment="Center"
                   Margin="5" /&gt;

        &lt;StackPanel Grid.Column="1" Grid.Row="0"
                    Spacing="5" Margin="5" &gt;

            &lt;!-- This button will ask HAL to open the doors --&gt;
            &lt;Button Command="{Binding OpenThePodBayDoorsDirectCommand}"
                    Content="Open the pod bay doors, HAL." /&gt;

            &lt;!-- This button will ask HAL to open the doors for a fellow robot --&gt;
            &lt;!-- We use CommandParameter to provide the name of the robot --&gt;
            &lt;!-- Note that the button is automatically disabled if we don't enter a name--&gt;
            &lt;StackPanel Orientation="Horizontal" Spacing="5"&gt;
                &lt;TextBox Text="{Binding RobotName}" Watermark="Robot Name" /&gt;
                &lt;Button Command="{Binding OpenThePodBayDoorsFellowRobotCommand}"
                        Content="{Binding RobotName, StringFormat='Open the Pod Bay for {0}'}"
                        CommandParameter="{Binding RobotName}" /&gt;
            &lt;/StackPanel&gt;

            &lt;!-- This button will ask start a multi-step sequence to open the doors --&gt;
            &lt;!-- As this is a long operation, the command is async.--&gt;
            &lt;!-- HAL inform us about the current progress of the doors.--&gt;
            &lt;!-- Note: The button will be disabled while the process is running. --&gt;
            &lt;Button Command="{Binding OpenThePodBayDoorsAsyncCommand}"
                    Content="Start Pod Bay Opening Sequence" /&gt;

        &lt;/StackPanel&gt;

        &lt;Separator Grid.Row="1" Grid.ColumnSpan="2"
                   HorizontalAlignment="Stretch"
                   Margin="5"
                   Height="2"
                   Background="LightGray"/&gt;

        &lt;TextBlock Grid.Column="0" Grid.Row="2"
                   Text="HAL 9000:"
                   FontWeight="Bold"
                   Margin="5"
                   VerticalAlignment="Center"/&gt;

        &lt;!-- This is the area where the output of HAL9000 is rendered --&gt;
        &lt;Border Grid.Column="1" Grid.Row="2"
                CornerRadius="10" Margin="5"
                BorderThickness="1" BorderBrush="{DynamicResource SystemAccentColor}"&gt;
            &lt;Grid ColumnDefinitions="*,Auto"&gt;
                &lt;ItemsControl Margin="5"
                              ItemsSource="{Binding ConversationLog}" /&gt;

                &lt;!-- This button will clear the recent conversation log--&gt;
                &lt;!-- which is not supported via CompiledBinding.                                 --&gt;
                &lt;Button Grid.Column="1"
                        VerticalAlignment="Stretch" VerticalContentAlignment="Center"
                        Background="Transparent"
                        Command="{Binding ConversationLog.Clear}"
                        Content="❌"/&gt;
            &lt;/Grid&gt;
        &lt;/Border&gt;
    &lt;/Grid&gt;
&lt;/UserControl&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>The <code>UserControl</code> itself will be added into the file <code>Views ► MainWindow</code> or <code>MainView</code>, depending on the template used to create this App.</p>
</div>
<div class="listingblock">
<div class="title">MainWindow.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;TabControl&gt;
	&lt;TabItem Header="ReactiveUI"&gt;
		&lt;views:ReactiveUiCommandsSampleView DataContext="{Binding ReactiveUiCommandsViewModel}" /&gt;
	&lt;/TabItem&gt;
&lt;/TabControl&gt;</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_7_see_it_in_action">Step 7: See it in action</h3>
<div class="paragraph">
<p>In your IDE press <code>Run</code> or <code>Debug</code> and try the sample out. Notice how the <code>Buttons</code> will be enabled and disabled automatically.</p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/command_sample_result.gif" alt="Result">
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_solution_2_use_communitytookit_mvvm">Solution 2: Use CommunityTookit-MVVM</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In this sample we will show you how to reduce the needed boilerplate code by using the source generators provided.</p>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
If you don&#8217;t know what a source generator does, you can learn more about it <a href="https://learn.microsoft.com/en-us/shows/on-dotnet/c-source-generators">[here]</a>
</td>
</tr>
</table>
</div>
<div class="sect2">
<h3 id="_step_1_add_a_viewmodel">Step 1: Add a ViewModel</h3>
<div class="paragraph">
<p>Add a new class into the <code>ViewModels</code>-folder called <code>CommunityToolkitCommandsViewModel</code>. This class should inherit <code>ViewModelBase</code> or <code>ObservableObject</code>.
Now open the file <code>ViewModels ► MainWindowViewModel.cs</code> or <code>MainViewModel</code> and add a new instance of the newly created class to it.</p>
</div>
<div class="admonitionblock warning">
<table>
<tr>
<td class="icon">
<i class="fa icon-warning" title="Warning"></i>
</td>
<td class="content">
Since our above model was using <code>ReativeUI</code>, our <code>ViewModelBase</code> inherits <code>ReactiveObject</code>. Therefore, we either need to change this or inherit <code>ObservableObject</code> directly.
</td>
</tr>
</table>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
We want to use source generators here. To make this work, our class must be marked as <code>partial</code>
</td>
</tr>
</table>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public partial class CommunityToolkitCommandsViewModel : ObservableObject
{
}</code></pre>
</div>
</div>
<div class="listingblock">
<div class="title">MainViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">public class MainViewModel : ViewModelBase
{
    public CommunityToolkitCommandsViewModel CommunityToolkitCommandsViewModel { get; } = new CommunityToolkitCommandsViewModel();
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>We want to store a record of our conversation with HAL-9000.  We use an <a href="https://docs.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.observablecollection-1?view=net-6.0"><code>[ObservableCollection]</code></a>, because this collection will notify out UI whenever a line was added or removed. In the file <code>CommunityToolkitCommandsViewModel</code> add the following lines:</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
///  This collection will store what the computer said
/// &lt;/summary&gt;
public ObservableCollection&lt;string&gt; ConversationLog { get; } = new ObservableCollection&lt;string&gt;();

// Just a helper to add content to ConversationLog
private void AddToConvo(string content)
{
    ConversationLog.Add(content);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Since HAL-9000 is not likely to listen to us, we also want to have the suggestion come
from a fellow robot. We want a field to put in the robot&#8217;s name. Let&#8217;s add a property:</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
/// The name of a robot. If the name is null or empty, there is no other robot present.
/// &lt;/summary&gt;
[ObservableProperty]
private string? _robotName;</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
Learn more about the <a href="https://learn.microsoft.com/de-de/dotnet/communitytoolkit/mvvm/generators/observableproperty">[<code>ObservableProperty</code>]</a>-attribute.
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_step_2_create_the_open_the_pod_bay_doors_command">Step 2: Create the "Open the POD bay doors" - Command</h3>
<div class="paragraph">
<p>In the <code>ViewModel</code>, we use the <a href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/relaycommand">[<code>RelayCommand</code>]</a> attribute to create commands.
The <a href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/relaycommand">[<code>RelayCommand</code>]</a> attribute instructs the MVVM Toolkit to automatically generate an <code>IRelayCommand</code> property that wraps the annotated method.
Even though you don’t write this property yourself, you can bind to it directly from XAML because it’s generated automatically at compile time.</p>
</div>
<div class="paragraph">
<p>Now we will create a method which will be executed when the <code>RelayCommand</code> is invoked.</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
/// The method that will be executed when the RelayCommand is invoked.
/// HAL responds to the human named "Dave" that he can't do that.
/// This is a famous line from the movie.
/// &lt;summary&gt;
[RelayCommand]
private void OpenThePodBayDoorsDirect()
{
    ConversationLog.Clear();
    AddToConvo( "I'm sorry, Dave, I'm afraid I can't do that.");
}</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_3_create_the_fellow_robot_command">Step 3: Create the "Fellow Robot" - Command</h3>
<div class="paragraph">
<p>Perhaps we will have more luck if the command is given by a fellow robot.</p>
</div>
<div class="paragraph">
<p>This <code>Command</code> is very similar to the one above. This time we will use a <code>CommandParameter</code> which is the name of another robot, but in fact it can be any <code>object</code> depending on your needs. We will also use the <code>CommandParameter</code> to enable or disable the <code>Command</code>, depending on the state of the parameter. Instead of initializing from the constructor, we will use the <a href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/relaycommand">[<code>RelayCommand</code>]</a>-attribute</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
/// This command will ask HAL to open the pod bay doors, but this time we
/// check that the command is issued by a fellow robot (really any non-null name)
/// &lt;/summary&gt;
/// &lt;remarks&gt;
/// We use the provided &lt;see href="https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/relaycommand"&gt;source generator&lt;/see&gt;,
/// since this is quite convenient. We just have to add the &lt;c&gt;[RelayCommand]&lt;/c&gt;-attribute.
/// Please visit the docs for more info about this.
/// &lt;/remarks&gt;
[RelayCommand(CanExecute = nameof(CanRobotOpenTheDoor))]
private void OpenThePodBayDoorsFellowRobot(string? robotName)
{
    ConversationLog.Clear();
    AddToConvo($"Hello {robotName}, the Pod Bay is open :-)");
}

/// &lt;summary&gt;
/// Check if the Robot can open the door (it needs to have a valid name)
/// &lt;/summary&gt;
/// &lt;returns&gt;true if the robot has a name&lt;/returns&gt;
private bool CanRobotOpenTheDoor() =&gt; !string.IsNullOrWhiteSpace(RobotName);</code></pre>
</div>
</div>
<div class="paragraph">
<p>As you can see above we have also introduced a method called <code>CanRobotOpenTheDoor</code> which is used to enable or disable the command. However, we need to actively trigger the command invalidation. That can be done via <code>MyCommand.NotifyCanExecuteChanged()</code> or via the <code>[NotifyCanExecuteChangedFor]</code>-attribute, which we will add to our <code>RobotName</code>-property:</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-diff" data-lang="diff">    [ObservableProperty]
+   [NotifyCanExecuteChangedFor(nameof(OpenThePodBayDoorsFellowRobotCommand))]
    private string? _robotName;</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_step_4_create_open_the_pod_bay_sequence_async_command">Step 4: Create "Open the Pod Bay sequence" - Async Command</h3>
<div class="paragraph">
<p>Many operations have multiple steps and we want to keep the user interface responsive while
we wait for sequence of operations to complete. One way to do this in <code>C#</code> is to use <a href="https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/">[<code>async / await</code>]</a> in combination with <a href="https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task?view=net-6.0">[<code>Tasks</code>]</a> to run things in the background while not blocking the UI.</p>
</div>
<div class="paragraph">
<p>Let&#8217;s add the <code>Command</code> and the <code>Task</code> to our <code>ViewModel</code>:</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsViewModel.cs</div>
<div class="content">
<pre class="highlight"><code class="language-csharp" data-lang="csharp">/// &lt;summary&gt;
/// This command will start an async task for the multi-step Pod bay opening sequence
/// &lt;/summary&gt;
/// &lt;remarks&gt;
/// This method is an async Task because opening the pod bay doors can take long time.
/// We don't want our UI to become unresponsive.
/// &lt;/remarks&gt;
[RelayCommand]
private async Task OpenThePodBayDoorsAsync()
{
    ConversationLog.Clear();
    AddToConvo("Preparing to open the Pod Bay...");
    // wait a second
    await Task.Delay(1000);

    AddToConvo("Depressurizing Airlock...");
    // wait 2 seconds
    await Task.Delay(2000);

    AddToConvo("Retracting blast doors...");
    // wait 2 more seconds
    await Task.Delay(2000);

    AddToConvo("Pod Bay is open to space!");
}</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
When you execute this command, notice that the <code>Button</code> is disabled until the <code>Task</code> is finished, so you can&#8217;t ask for the pod bay doors to open again while the opening sequence is running. A more complete program could keep track of the state of the Pod Bay and change the button to "close".
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<div class="sect2">
<h3 id="_step_5_setting_up_the_user_interface">Step 5: Setting up the User Interface</h3>
<div class="paragraph">
<p>Add a new <code>UserControl</code> into the <code>Views</code> folder called <code>CommunityToolkitCommandsSampleView</code>. The content will be the same as in the above sample, see <a href="#Solution1-Step6">Solution 1, Step 6</a></p>
</div>
<div class="paragraph">
<p>Only thing we need to change is the <code>ViewModel</code>-reference:</p>
</div>
<div class="listingblock">
<div class="title">CommunityToolkitCommandsSampleView.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-diff" data-lang="diff">&lt;UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:viewModels="clr-namespace:CommandSample.ViewModels"
             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
!            x:DataType="viewModels:CommunityToolkitCommandsViewModel"
             x:Class="CommandSample.Views.CommunityToolkitCommandsSampleView"&gt;

    &lt;Design.DataContext&gt;
!       &lt;viewModels:CommunityToolkitCommandsViewModel/&gt;
    &lt;/Design.DataContext&gt;</code></pre>
</div>
</div>
<div class="paragraph">
<p>The <code>UserControl</code> itself will be added into the file <code>Views ► MainWindow</code> or <code>MainView</code>, depending on the template used to create this App.</p>
</div>
<div class="listingblock">
<div class="title">MainWindow.axaml</div>
<div class="content">
<pre class="highlight"><code class="language-xml" data-lang="xml">&lt;TabControl&gt;
	&lt;TabItem Header="ReactiveUI"&gt;
		&lt;views:ReactiveUiCommandsSampleView DataContext="{Binding ReactiveUiCommandsViewModel}" /&gt;
	&lt;/TabItem&gt;
    &lt;TabItem Header="CommunityToolkit-MVVM"&gt;
        &lt;views:CommunityToolkitCommandsSampleView DataContext="{Binding CommunityToolkitCommandsViewModel}" /&gt;
    &lt;/TabItem&gt;
&lt;/TabControl&gt;</code></pre>
</div>
</div>
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<div class="sect2">
<h3 id="_step_6_see_it_in_action">Step 6: See it in action</h3>
<div class="paragraph">
<p>Refer to <a href="#_step_7_see_it_in_action">Step 7: See it in action</a></p>
</div>
<div class="imageblock">
<div class="content">
<img src="_docs/command_sample_result.gif" alt="Result">
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_related">Related</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Beside the options shown above, you can also try out other <code>ICommand</code> implementations. Here are a few to try:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="https://prismlibrary.com/docs/commands/commanding.html">[DelegateCommand]</a> from <code>Prism Library</code></p>
</li>
<li>
<p><a href="https://docs.avaloniaui.net/docs/guides/data-binding/how-to-bind-to-a-command-without-reactiveui">[Avalonia can bind to methods]</a></p>
</li>
</ul>
</div>
</div>
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</div>
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Last updated 2025-11-07 18:43:23 UTC
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